1. Field of the Invention
The present invention relates to gas discharge display and memory devices. More particularly, the present invention relates to light pen detection and tracking methods and apparatus for detecting the position and movement of a light pen on the surface of an AC gas discharge display and memory panel.
2. Description of the Prior Art
Gas discharge display and memory panels of the type to which the present invention pertains are well known in the art. For example, U.S. Pat. No. 3,499,167 to Baker et al describes such a panel. The gas panels of the type to which the present invention pertains typically have two glass plates maintained in spaced-apart relationship, and are arranged to have sealed between the spaced-apart plates an ionizable medium. To provide matrix addressability whereby selected local regions within the ionizable medium may be selectively ionized, sets of horizontal and vertical conductors are employed. Typically, the set of horizontal conductors comprises an array of parallel insulated conductors arranged on the inner surface of one plate and horizontally extending thereacross. Likewise, the set of vertical conductors comprises an array of parallel insulated conductors arranged on the inner surface of the other plate vertically extending thereacross, generally orthogonal to the horizontal conductors.
In such arrangements, when an appropriate voltage is applied between a selected one of the horizontal conductors and a selected one of the vertical conductors, ionization occurs at the crossover point of the two conductors, such that light is emitted. Generally, the crossover points are referred to as "cells" and a display pattern or image is formed by ionizing selected cells. For another example of a panel as described and to which the present invention pertains, reference is made to an article by D. L. Bitzer et al entitled "The Plasma Display Panel -- A Digitally Addressable Display with Inherent Memory," Proceedings of the Fall Joint Computer Conference IEEE, San Francisco, California, November 1966, pp. 541-547.
Although light pen detection and tracking functions have been implemented in prior art AC gas discharge display and memory panels, such prior art implementations have been found to be inadequate for a variety of reasons. Typically, the prior art light pen detection and tracking approaches involve disturbing information stored in the panel. One form of panel information disturbance is that utilized by Tucker in U.S. Pat. No. 3,852,721. In Tucker, light pen tracking is achieved by positioning a cursor on the display panel. Displaying the cursor necessitates the storing of the information overwritten in the plasma panel by the cursor until the cursor has moved on and then rewriting the stored information in its original location. Obviously, such an approach necessitates the use of memory (in addition to the panel memory) for purposes of regenerating the former information state lost as a result of the formation of the cursor. In addition, such an approach necessarily involves additional control circuitry for moving the information back and forth between the memory and panel.
Another approach to light pen detection and tracking involves a partial disturbance of the memory state of the cells of the panel. Such partial disturbance approaches endeavor to detect the light pen location without destroying the memory state of the pattern of "on" and "off" cells in the panel. Since the condition of the cells in the "on" state is significantly different from the cells in the "off" state, the partial disturbance approach necessitates separate treatment of the "on" cells from the "off" cells. For example, U.S. Pat. No. 3,887,767 to Miller describes a light pen detection scheme wherein several steps are involved to first disturb the "off" cells to a degree to permit their detection without the loss of the state, and then to disturb the "on" cells to a degree sufficient to permit their detection without the loss of their memory state. Miller effects his detection processes in two passes of the panel, the first to disturb the "off" state cells and the second to disturb the "on" cells. In the latter instance, the coordinates of cells in the "on" state of discharge are identified by inverting the device stored states to place them to the "off" state, employing the same interrogation sequence as is employed to detect the "off" state cells and then reinverting the stored states.
Another prior art partial disturbance approach is that described by Ngo in U.S. Pat. No. 3,851,327 and SID articles entitled, respectively, "Light Pen Capability on Plasma Display Panel," Digest of Technical Papers, 1974 SID International Symposium, May 1974, pp. 24 and 25, and "Dynamic Light Pen Tracking on a Plasma Panel," Digest of Papers, 1974 SID International Symposium, May 1974, pp. 26 and 27. In general, the above-cited Ngo patent and articles describe a technique for detecting a position of a light pen over lighted areas, i.e., "on" cells of an AC plasma panel.
A further article by Ngo entitled "Light Pen Detection Over Dark Areas of an AC Plasma Panel," Digest of Technical Papers, 1975 SID International Symposium, pp. 110 and 111, describes a technique for detecting the position of a light pen over dark areas, i.e., "off" cells of an AC plasma panel. One of the problems encountered in detecting "off" cells by partial disturbance is wall charge build-up in the dark areas. Because of the wall charge build-up problem, constraints are necessarily imposed upon systems using a partial disturbance approach, and affirmative steps necessarily have to be implemented to obviate this problem.
One further difficulty with the Ngo approach resides in the fact that the delayed selective erase pulse used to detect "on" cells may act to reduce the panel sustain margin. In order to obviate this problem, Criscimagna et al describe in an article entitled "Light-Pen Detection with a Plasma Display Panel," IEEE Transactions on Electron Devices, Vol. 22, September 1975, pp. 796-799, an approach wherein the sustain segment is used as the selective erase disturb pulse. Basically, Criscimagna et al utilize a disturb pulse made up of an additional sustain alteration, selectively applied to each cell or line to move the light output of a selected cell ahead in time.
Another approach to light pen detection is that described by Schermerhorn in U.S. Pat. No. 3,875,472. In general, the Schermerhorn approach involves a multicell device having addressable subsites in each cell. By using addressable subsites, the panel may be selectively scanned with signals to address less than all of the subsites, whereby detection is achieved.
The difficulty with all of the above-described light pen detection techniques resides in the fact that they are complex and cumbersome, requiring multiple procedures and/or apparatus. In general, such approaches are costly, in that they necessitate a special apparatus and/or impose constraints on the operating characteristics and conditions of conventional AC gas discharge display panels.
It is, therefore, an object of the present invention to provide an improved AC gas discharge display system.
It is yet a further object of the present invention to provide an improved light pen detection and tracking method and apparatus for AC gas discharge plasma display panels.
It is yet still a further object of the present invention to provide a light pen detection and tracking scheme for plasma display panels which is simple in design and effective in operation.
It is another object of the present invention to provide a light pen detection and tracking arrangement for AC plasma display panels which is rapid in response, i.e., can follow a light pen in real time without the necessity of a complex tracking strategy.
It is still another object of the present invention to provide a light pen detection and tracking arrangement for plasma display panels which permits nondestructive scanning of the panel in a single detection cycle to detect both "on" and "off" cells using a single waveform complex in phase with the normal sustain cycle such that conventional plasma display panels and drive circuitry may be utilized.
It is yet still another object of the present invention to provide a light pen detection and tracking arrangement for plasma display panels wherein both "on" and "off" cells are nondestructively scanned in a single pass by a special scan waveform complex which acts, in a single cycle, to fully change the respective states of the panel cells and then to fully return such cells to their original state.
It is yet still another and further object of the present invention to provide a light pen detection and tracking arrangement for an AC gas display panel which utilizes the same waveform complex as may be used to form a cursor at the detected location of the light pen.